U.S. patent application number 12/067204 was filed with the patent office on 2009-12-17 for displaced electrode amplifier.
This patent application is currently assigned to Halliburton Energy Servies, Inc.. Invention is credited to George D. Goodman, Edward J. Harros.
Application Number | 20090309591 12/067204 |
Document ID | / |
Family ID | 38049358 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090309591 |
Kind Code |
A1 |
Goodman; George D. ; et
al. |
December 17, 2009 |
DISPLACED ELECTRODE AMPLIFIER
Abstract
A displaced electrode amplifier ("DEA") for measuring signals
from high impedance sources. The amplifier may include an
operational amplifier ("op-amp") configured as a unity gain buffer,
with a feedback path to the non-inverting input to at least partly
compensate for a parasitic input shunt impedance. In cases where
the device is to measure AC signals in high ambient temperatures,
the non-inverting input may be coupled via a large resistance to a
ground reference that is driven with a second feedback signal to
magnify the effective value of the large resistance. Where a
differential configuration is desired, one or more tuning resistors
may be provided to match responses of different input buffer
stages, thereby maximizing the common mode rejection. The disclosed
amplifier is suitable for use in oil-based mud resistivity imaging
tools but is also suitable for other applications.
Inventors: |
Goodman; George D.;
(Phoenixville, PA) ; Harros; Edward J.;
(Pottstown, PA) |
Correspondence
Address: |
KRUEGER ISELIN LLP (1391)
P O BOX 1906
CYPRESS
TX
77410-1906
US
|
Assignee: |
Halliburton Energy Servies,
Inc.
Houston
TX
|
Family ID: |
38049358 |
Appl. No.: |
12/067204 |
Filed: |
November 10, 2006 |
PCT Filed: |
November 10, 2006 |
PCT NO: |
PCT/US06/60774 |
371 Date: |
March 18, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60736105 |
Nov 10, 2005 |
|
|
|
Current U.S.
Class: |
324/303 ;
324/123R; 330/97 |
Current CPC
Class: |
G01V 3/24 20130101 |
Class at
Publication: |
324/303 ; 330/97;
324/123.R |
International
Class: |
G01V 3/00 20060101
G01V003/00; H03F 1/00 20060101 H03F001/00; G01R 1/30 20060101
G01R001/30 |
Claims
1. A displaced electrode amplifier that comprises: an operational
amplifier ("op-amp") having an inverting input, a non-inverting
input, and an output, wherein the output is coupled to the
inverting input to configure the op-amp as a unity gain buffer; and
a feedback impedance coupled between the output and the
non-inverting input to at least partly compensate for a parasitic
shunt impedance coupled to the non-inverting input.
2. The displaced electrode amplifier of claim 1, wherein the
feedback impedance comprises a series combination of a capacitor
and a resistor.
3. The displaced electrode amplifier of claim 1, further
comprising: an input impedance coupled between the non-inverting
input and a sensing electrode.
4. The displaced electrode amplifier of claim 3, wherein the input
impedance comprises a series combination of a capacitor and a
resistor.
5. The displaced electrode amplifier of claim 1, further
comprising: an electrically conductive shield for capacitance
guarding the non-inverting input to a sensing electrode; and a
resistor coupled between the output and the electrically conductive
shield.
6. The displaced electrode amplifier of claim 1, further comprising
a large resistor coupled between the non-inverting input and a
reference node.
7. The displaced electrode amplifier of claim 6, wherein the
reference node is driven by a second op-amp having its output
coupled to its inverting input, and having its non-inverting input
coupled to ground via a second resistor.
8. The displaced electrode amplifier of claim 7, wherein the
non-inverting input of the second op-amp is further coupled to the
output of the first op-amp via a capacitor to drive the reference
node with positive feedback to magnify the effective value of the
large resistor.
9. The displaced electrode amplifier of claim 3, further
comprising: a second op-amp having an inverting input, a
non-inverting input and an output, wherein the output of the second
op-amp is coupled to the inverting input of the second op-amp to
configure the second op-amp as a unity gain buffer; a second
feedback impedance coupled between the out-put of the second op-amp
and the non-inverting input of the second op-amp to at least partly
compensate for a parasitic input shunt impedance; and a
differential amplifier stage coupled to the outputs of the first
and second op-amps to produce an amplified difference signal.
10. The displaced electrode amplifier of claim 9, wherein the
differential amplifier stage includes a reference resistor coupled
between the outputs of the first and second op-amps.
11. The displaced electrode amplifier of claim 10, wherein the
differential amplifier further includes a tuning resistor coupled
between a reference voltage and one of the outputs of the first and
second op-amps, wherein the tuning resistor has a value selected to
provide a null in a common mode rejection response of the displaced
electrode amplifier.
12. An oil-based mud imaging tool that comprises: a sensor array
having one or more voltage electrodes and one or more current
electrodes, wherein the one or more current electrodes are
energized by an excitation source to create an oscillatory current
flow in a borehole wall; and at least one displaced electrode
amplifier coupled to the one or more voltage electrodes to measure
a differential voltage created by the oscillatory current flow,
wherein the displaced electrode amplifier employs positive feedback
to nullify parasitic input shunt impedances of the voltage
electrodes.
13. The tool of claim 12, wherein the displaced electrode amplifier
comprises: two input buffer stages each coupled to a respective one
of the voltage electrodes to provide the positive feedback; and a
differential amplifier stage coupled to the input buffer stages to
measure the differential voltage.
14. The tool of claim 13, wherein the differential amplifier stage
also operates to match the responses of the input buffer stages in
a frequency range of interest.
15. The tool of claim 13, wherein each of the input buffer stages
includes a high impedance reference voltage that is not susceptible
to bias currents that commonly develop under high operating
temperatures.
16. The tool of claim 14, wherein each of the input buffer stages
drives the high impedance ground reference with positive feedback
to reduce loading in a frequency range of interest.
17. The tool of claim 13, wherein each of the input buffer stages
further includes an electrically conductive shield for wiring to
the corresponding voltage electrode, and wherein the electrically
conductive shield is driven from an output signal of the input
buffer stage.
18. A voltage sensing method that comprises: receiving a signal
from a high impedance source via an input electrode; providing a
high-pass filtered version of the signal to an input node;
buffering a voltage from the input node to produce an output
signal; providing a feedback signal to the input node to compensate
for a parasitic input shunt impedance; and processing the output
signal to determine a measurement for storage or display.
19. The method of claim 18, further comprising: coupling the input
node to a high impedance reference node.
20. The method of claim 19, further comprising: driving the high
impedance reference node with a second feedback signal to increase
an effective value of a resistor that couples the input node to the
high impedance reference node.
21. The method of claim 18, wherein said processing includes:
combining the output signal with a second output signal derived
from a second input node to produce a differential signal; and
employing at least one tuning resistor to provide maximal common
mode rejection.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to provisional U.S.
Patent Application 60/736,105, filed Nov. 10, 2005 and entitled
"Displaced Electrode Amplifier", which is hereby incorporated
herein by reference.
BACKGROUND
[0002] Modern oil field operations demand a great quantity of
information relating to the parameters and conditions encountered
downhole. Such information typically includes characteristics of
the earth formations traversed by the borehole, and data relating
to the size and configuration of the borehole itself. The
collection of information relating to conditions downhole, which
commonly is referred to as "logging," can be performed by several
methods including wireline logging and "logging while drilling"
(LWD).
[0003] In both wireline and LWD environments, it is often desirable
to construct an image of the borehole wall. Among other things,
such images reveal the fine-scale structure of the penetrated
formations. The fine-scale structure includes stratifications such
as shale/sand sequences, fractures, and non-homogeneities caused by
irregular cementation and variations in pore size. Orientations of
fractures and strata can also be identified, enabling more accurate
reservoir flow modeling.
[0004] Borehole wall imaging can be accomplished in a number of
ways, but micro-resistivity tools have proven to be effective for
this purpose. Micro-resistivity tools measure resistivity of the
borehole surface on a fine scale. The resistivity measurements can
be converted into pixel intensity values to obtain a borehole wall
image. However, oil-based muds can inhibit such measurements due to
the variability of impedance in the mud surrounding the tool.
[0005] U.S. Pat. No. 6,191,588 (Chen) discloses an imaging tool for
use in oil-based muds. Chen's resistivity tool employs at least two
pairs of voltage electrodes positioned on a non-conductive surface
between a current source electrode and a current return electrode.
At least in theory, the separation of voltage and current
electrodes eliminates the oil-based mud's effect on voltage
electrode measurements, enabling at least qualitative measurements
of formation resistivity based on the injection of a current
excitation signal and the subsequent measurement of the voltage
drop across the formation. The voltage drop sensed between the
voltage electrodes is amplified, conditioned, acquired, and used
with a measured current flow to calculate an estimate of formation
resistivity in front of the pad.
[0006] The implementation of a differential amplifier to measure a
signal corresponding to the voltage drop in the formation
encounters several obstacles. These obstacles include: the
limitations inherent in the circuitry, the interactions of the
sensor pad with the surrounding environment, the properties and
standoff thickness of the mud, and the tilt angle of the pad
relative to the formation. Among other things, these obstacles
create a vulnerability to measurement error due to a common mode
signal at the voltage electrodes relative to the amplifier
reference ground. The above-named obstacles contribute variability
to the voltage dividers defined by the input impedance of the
amplifier and the impedances between the voltage electrodes and the
formation. The finite input impedance of the amplifier circuit
allows a small amount of current flow through these variable
voltage dividers, converting the common mode voltage into a
differential voltage component at the voltage electrodes.
[0007] Significant effort has been made to minimize the effects of
common mode voltage. For example, one proposed method of reducing
the common mode voltage relies on isolating the current source
transmitter circuitry from the reference ground of the amplifier.
However, attempts to provide a high-impedance isolation for the
amplifier have been largely unsuccessful, and the residual
sensitivity of the measurement circuitry to the common mode voltage
remains too high to gather accurate measurements in boreholes
having an oil-based mud.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] In the following detailed description, reference will be
made to the accompanying drawings, in which:
[0009] FIG. 1 shows an illustrative logging while drilling (LWD)
environment;
[0010] FIG. 2 shows an illustrative wireline logging
environment;
[0011] FIG. 3 shows an illustrative first logging tool
configuration;
[0012] FIG. 4 shows an illustrative second logging tool
configuration;
[0013] FIG. 5 shows a front view of an illustrative sensor pad;
[0014] FIG. 6 shows a cross section of the illustrative sensor
pad;
[0015] FIG. 7 shows a circuit diagram to illustrate operating
principles;
[0016] FIG. 8 is a flow diagram of an illustrative imaging
method;
[0017] FIG. 9 is a model of various contributors to measurement
error;
[0018] FIG. 10 is a circuit diagram of an input buffer stage;
[0019] FIG. 11 is a circuit diagram of a differential amplifier
stage; and
[0020] FIG. 12 is an electrical schematic of a displaced electrode
amplifier in accordance with certain preferred embodiments.
[0021] The drawings show illustrative invention embodiments that
will be described in detail. However, the description and
accompanying drawings are not intended to limit the invention to
the illustrative embodiments, but to the contrary, the intention is
to disclose and protect all modifications, equivalents, and
alternatives falling within the spirit and scope of the appended
claims.
DETAILED DESCRIPTION
[0022] Disclosed herein are various methods and apparatuses for
accurately sensing a voltage potential through a dielectric layer.
These methods and apparatuses are applicable to logging instruments
and systems for imaging in nonconductive fluids such as an
oil-based mud. In some embodiments, the disclosed methods and
apparatuses are employed in logging systems having a logging tool
in communication with surface computing facilities. The logging
tool includes a sensor array having at least two voltage electrodes
positioned between at least two current electrodes. The current
electrodes inject an excitation signal into a formation forming
part of a borehole wall. A displaced electrode amplifier ("DEA")
circuit is coupled to the voltage electrodes to measure a
differential voltage between the voltage electrodes. The amplifier
circuit includes input buffers having feedback to compensate for
parasitic elements inherent in the measurement circuitry. As a
result, the amplifier's input resistance is markedly increased, and
the amplifier's input capacitance is significantly lowered beyond
currently available configurations of comparable complexity.
Moreover, the amplifier circuit includes tuning resistors that
provide greatly reduced sensitivity to common mode voltage
signals.
[0023] FIG. 1 shows an illustrative logging while drilling (LWD)
environment in which a tool having the disclosed amplifier may be
employed. A drilling platform 2 supports a derrick 4 having a
traveling block 6 for raising and lowering a drill string 8. A
kelly 10 supports the drill string 8 as it is lowered through a
rotary table 12. A drill bit 14 is driven by a downhole motor
and/or rotation of the drill string 8. As bit 14 rotates, it
creates a borehole 16 that passes through various formations 18. A
pump 20 circulates drilling fluid through a feed pipe 22 to kelly
10, downhole through the interior of drill string 8, through
orifices in drill bit 14, back to the surface via the annulus
around drill string 8, and into a retention pit 24. The drilling
fluid transports cuttings from the borehole into the pit 24 and
aids in maintaining the borehole integrity.
[0024] An LWD resistivity imaging tool 26 is integrated into the
bottom-hole assembly near the bit 14. As the bit extends the
borehole through the formations, logging tool 26 collects
measurements relating to various formation properties as well as
the bit position and various other drilling conditions. The logging
tool 26 may take the form of a drill collar, i.e., a thick-walled
tubular that provides weight and rigidity to aid the drilling
process. A telemetry sub 28 may be included to transfer tool
measurements to a surface receiver 30 and to receive commands from
the surface receiver.
[0025] At various times during the drilling process, the drill
string 8 may be removed from the borehole. Once the drill string
has been removed (as shown in FIG. 2), logging operations can be
conducted using a wireline logging tool 34, i.e., a sensing
instrument sonde suspended by a cable 42 having conductors for
transporting power to the tool and telemetry from the tool to the
surface. A resistivity imaging portion of the logging tool 34 may
have sensing pads 36 that slide along the borehole wall as the tool
is pulled uphole. A logging facility 44 collects measurements from
the logging tool 34, and includes computing facilities for
processing and storing the measurements gathered by the logging
tool. The computing facilities may take the form of a personal
computer, server, or digital signal processing board, or some other
form of computing circuit.
[0026] In both the LWD and wireline forms, the resistivity imaging
tool include electrode arrays for coupling the displaced electrode
amplifier to the borehole wall. FIG. 3 shows a cross-sectional view
of LWD resistivity imaging tool 26 in a borehole 16. A biasing
mechanism 302 de-centralizes tool 26 to minimize the standoff
between the tool's sensors and the borehole wall. The tool's
electrode array(s) may be located in a pad on biasing mechanism
302, or alternatively they may be located in the main body of the
tool opposite the biasing mechanism. As the tool 26 rotates and
progresses downhole at the drilling rate, the electrode arrays will
trace a helical path on the borehole wall. Orientation sensors
within the tool may be used to associate the resistivity
measurements with the electrode positions on the borehole wall.
Surface computing facilities may collect resistivity measurements,
orientation (azimuth) measurements, and tool position measurements,
and may process the collected measurements to create a resistivity
image of the rock formation surrounding the borehole.
[0027] FIG. 4 shows a cross-sectional view of one embodiment of the
wireline resistivity imaging tool 34 in a borehole 16, which may
also represent an alternative configuration for the LWD resistivity
imaging tool 26. Sensing pads 36 are deployed against the borehole
wall to minimize standoff. Multiple pads may be used to obtain
measurements over a greater fraction of the borehole's
circumference. In some embodiments, the pads are provided in
axially-offset groups to increase circumferential coverage without
undue crowding in the undeployed configuration.
[0028] In the logging scenarios described above with respect to
FIGS. 1 and 2, the drilling fluid present in the borehole is an
electrically nonconductive fluid such as an oil-based mud. Some of
the fluid may mix with drill cuttings or material from the borehole
walls to form a viscous semi-solid layer on the borehole walls.
This layer is commonly termed "mudcake," and it prevents intimate
contact between logging sensors and uncontaminated formation
material. In addition, motion of the logging instruments may create
a fluid flow layer that further separates the logging sensors from
the uncontaminated formation materials.
[0029] FIG. 5 shows the face of an illustrative sensor pad 502
having six pairs of voltage electrodes 504 positioned between
current electrodes 506 and 508. In practice, the sensor pads may be
provided with additional voltage and current electrodes, and in
fact may operate on multiple axes. With uni-axial sensor pads such
as pad 502, the length of the sensor pad is kept parallel to the
long axis of tool 34. The distance between the current electrodes
506, 508 controls the depth of investigation, with greater
distances providing greater depths of investigation. The distances
between the voltage electrodes 504 control the spatial resolution
of the tool, with smaller distances providing higher
resolutions.
[0030] A cross-section of the illustrative sensor pad 502 is shown
in FIG. 6. Sensor pad 502 comprises a metal substrate 602 to
provide the pad with the needed rigidity and strength. The metal
substrate 602 may include cavities 604 to hold sensor circuitry.
For illustrative purposes, the electrode feeds are shown passing
through the sensor pad 502, but the electrode feeds may
alternatively connect to the sensor circuitry in cavities 604 or in
a central cavity (not shown). In some embodiments, metal substrate
602 comprises steel. The face of metal substrate 602 is covered
with an insulating layer 606, which in some embodiments comprises a
rubber or polyetheretherketone (PEEK) material. Current electrodes
506 and 508 are embedded on the face of the insulating layer
606.
[0031] FIG. 7 illustrates the principles behind the operation of
the resistivity imaging tool. Sensor pad 502, when pressed against
a borehole wall, is separated from the formation 702 by a
low-conductivity layer 704 of mudcake. An excitation source 706
injects an alternating current (AC) excitation signal 708 into the
formation via current electrodes 506 and 508. The excitation signal
708 creates a voltage drop along the current flow lines in the
formation, which can be measured via voltage electrodes C and D. A
differential amplifier 510 amplifies (and filters) the voltage
difference between the voltage electrodes and provides a
measurement signal for acquisition by an analog-to-digital
converter. Measurements of the current flow from either (or both)
of the current electrodes may be combined with the measured voltage
signal to determine a resistivity of the formation in front of the
voltage electrodes. The voltage and/or resistivity measurements may
be stored for later use and may be communicated to the surface and
displayed to a user. In computerized systems, the measurements may
be stored in a computer memory or in a long-term information
storage device such as a hard disk.
[0032] A method for using the resistivity imaging tool is described
in FIG. 8. In block 802, the resistivity imaging tool is placed in
a borehole. For LWD, the tool is part of the bottom hole assembly
to perform logging as drilling operations are performed. For
wireline logging, the tool is part of a sonde that is lowered to
the bottom of the region of interest to perform logging as the
logging tool is pulled uphole at a steady rate.
[0033] In block 804, the tool is placed in logging mode. For LWD,
this operation may (or may not) involve deploying a de-centralizer
that forces sensors in the tool body against the borehole wall.
Alternatively, the LWD resistivity imaging tool may have one or
more sensor pads that are deployed against the borehole wall. For
wireline logging, multiple sensor pads are deployed against the
borehole wall. Blocks 806-814 represent operations that occur
during the logging process. Though shown and described in a
sequential fashion, the various operations may occur concurrently,
and moreover, they may simultaneously occur for multiple voltage
electrode pairs and multiple sensor pads.
[0034] In block 806, the tool measures the current(s) through the
current electrodes, and further measures the voltage difference
between the various voltage electrode pairs. In block 808, the tool
determines a resistivity measurement for each voltage electrode
pair, e.g., by dividing the measured voltage difference by the
measured current. In block 810, the tool, or more likely, the
surface logging facility coupled to the tool, associates the
compensated resistivity measurements with a tool position and
orientation measurement, thereby enabling a determination of image
pixel values for imaging the lock formation surrounding the
borehole.
[0035] In block 812, the tool moves along the borehole, and in
block 814, a check is performed to determine whether logging
operations should continue (e.g., whether the logging tool has
reached the end of the region of interest). For continued logging
operations, blocks 806-914 are repeated Once logging operations are
complete (or in some embodiments, while the logging operations are
ongoing), the surface logging facility maps the resistivity
measurements into borehole wall image pixels and displays the
resulting resistivity image of the surrounding formations in block
816.
[0036] The implementation of differential amplifier 510 encounters
several performance obstacles in the above-described context. The
obstacles include inherent limitations of the circuitry, as well as
interactions with the environment, mudcake thickness and
properties, and the relative tilt angle of the pad relative to the
formation. The high impedance and variability of the mudcake layer
creates sensitivity to the common mode signal that could be present
at voltage electrodes C and D relative to the amplifier reference
ground. (The variability of the impedance between the voltage
electrodes (C or D) and the excitation source, together with the
finite input impedance of the amplifier, creates a variable voltage
divider which can convert the common mode voltage present in the
formation to a differential voltage component at the voltage
sensing electrodes.)
[0037] FIG. 9 illustrates the major parasitic effects that degrade
measurement quality even when an ideal operational amplifier
("op-amp") is used. A current source I2 represents the excitation
signal that is applied to the formation, which in turn is
represented by resistors R12 and R18 (shown with a value of
50.OMEGA. each). A voltage source V2 applied to the node between
resistors R12 and R18 represents the common mode voltage. A
parallel capacitor C8 and resistor R11 (shown as 0.55 pF and 642
k.OMEGA., respectively) represent the complex impedance of the
mudcake layer separating the first voltage electrode from the
formation, and the parallel arrangement of capacitor C9 and
resistor R10 (shown as 0.7 pF and 513 k.OMEGA.) represent the
mudcake layer separating the second voltage electrode from the
formation. Capacitor C7 (shown as 1.7 pF) represents the capacitive
coupling between the voltage electrodes, while capacitors C12 and
C13 (shown as 3 pF and 2 pF) represent the capacitance of the
wiring and amplifier inputs coupled to the voltage electrodes.
[0038] The equivalent circuit elements representing the mudcake
impedance can vary over a wide range of values. Mismatches in the
mudcake impedance, taken together with the shunt capacitances (C7,
C12, and C13), make the voltage measurement susceptible to error
from the common mode voltage component. The finite input impedance
(represented by the shunt capacitances) allows current to flow
across the mudcake layer, causing unequal voltage drops in the mud
layer in front of the voltage electrodes and adversely affecting
the quality of the measurements. As a result, an erroneous and
unwanted voltage differential is created between voltage electrodes
and is superimposed on the desired voltage difference measurement.
The effect of this measurement error may be particularly
significant during the measurement of formations with a low
resistivity (i.e., less than 5 Ohm-m) using the current injection
method described above, because the formation voltage drop to be
measured is relatively small.
[0039] To combat the current flow allowed by the finite input
impedance, a novel circuit configuration is presented having a
buffered input with feedback to reduce loading and to increase the
effective input impedance. FIG. 10 shows an input buffer stage 1002
having an input node M1U for coupling to a voltage electrode. Each
voltage electrode is preferably coupled to a corresponding input
buffer stage. The signal from the voltage electrode input node M1U
is high-pass filtered by a series combination of a capacitor C8 and
resistor R3 before reaching input node N1. This filtering blocks
currents that are naturally occurring in earth formations. (As
explained below, the input node N1 is supplied with positive
feedback, and accordingly the component values of capacitor C8 and
resistor R3 are preferably selected to assure unconditional
stability for all operating conditions.) Input node N1 is coupled
to the noninverting input of op-amp U1. Op-amp U1 has its output
coupled to its inverting input, which configures the op-amp to
operate as a non-inverting unity gain buffer.
[0040] The output of op-amp U1 is coupled to the output node VU,
but is also coupled via a positive feedback path to input node N1
to null the equivalent parasitic input capacitance, i.e., the stray
capacitance resulting from the input capacitance of op-amp U1, the
wiring, the sensor pad, and the tool body. The feedback path
includes a series combination of a resistor R125 and a capacitance
C4. The values of these components (R125 and C4) are selected in
combination with the values of components C8 and R3 to provide
maximum gain flatness in the frequency range of interest while
operating with maximum source impedance condition. It is
specifically noted that a high degree of flexibility exists in
choosing component values for the feedback path, and indeed, some
embodiments may omit either or both of the components R125 and C4
in favor of a short circuit (e.g., R125 may have a value of 0 ohms,
and C4 may be infinite). In tools having multiple voltage electrode
pairs ("channels"), the gain for each channel may vary due to the
placement of the voltage electrodes. The values of the feedback
path components may be adjusted to provide good gain matching
between channels. It is further noted that the values of the
feedback path components can be adjusted to match (or "tune") the
responses of the input buffer stages.
[0041] To reduce leakage currents due to stray capacitance, the
voltage electrode input node M1U, input node N1, and components C8,
R3, and C4 are shielded with a conductive shield or "guard
electrode" G1U. The output node VU is coupled to the guard
electrode G1U via a resistor R121 to keep the guard electrode at
about the same potential as the shielded nodes and components.
Because the guard electrode is capacitively coupled to the input
node N1 (thereby creating a second feedback path), resistor R121 is
provided to preclude instability.
[0042] Op-amp U16 has an output that is coupled to its inverting
input, configuring it to operate as a voltage follower. The output
is further coupled to input node N1 via a large resistor R1 to
perform two functions. At zero frequency (aka direct current, or
"DC"), the non-inverting input of op-amp U16 is coupled to ground
via resistor R127, causing op-amp U16 to provide a high impedance
(.about.200 k.OMEGA.) ground reference for op-amp U1, which is
desirable for operation in high ambient temperatures. At the
frequency range of interest, capacitors C107 and C109 couple the
non-inverting input of op-amp U16 between ground and the output
node VU, acting as a voltage divider. In the frequency range of
interest, the output of op-amp U16, though scaled, follows the
output node voltage, effectively increasing the value of R1 by
about 500 times. Thus, this secondary positive feedback path
minimizes loading effects for AC voltage measurements.
[0043] The output node of input buffer stage 1002 can be coupled to
a conventional amplifier for single-element use. For differential
amplification, the output nodes from input buffer stage 1002 and a
second such input buffer stage may be coupled to a differential
amplifier stage 1102 such as that shown in FIG. 11. Working
backwards, the output node Vout is coupled to ground via a
capacitor C104 and coupled to the output of op-amp U2 via resistor
R119. In this configuration, resistor R119 and capacitor C104 act
as a low pass filter to smooth the differential voltage
measurements and prevent aliasing during analog-to-digital
conversion.
[0044] The output of op-amp U2 is coupled to its inverting input
via a first impedance formed by a parallel combination of capacitor
CX3 and resistor R7, and the inverting input in turn is coupled via
a second impedance to the output node VU of an input buffer stage.
The second impedance is formed by a series combination of capacitor
CX1 and resistor R8. The output node VL of the other input buffer
stage is coupled to the non-inverting input of op-amp U2 via a
series combination of capacitor CX2 and resistor R9, which together
provide an impedance value equal to the second impedance. The
non-inverting input of op-amp U2 is further coupled to ground via a
parallel combination of capacitor CX4 and resistor R118, which
together provide an impedance value equal to the first impedance.
Configured in this manner, op-amp U2 is designed to produce an
output signal that amplifies the difference between the input
signals by a gain equal to the ratio of the first impedance to the
second impedance. The impedance values may be chosen to provide a
bandpass frequency response that passes the frequency range of
interest.
[0045] It is noted that the frequency responses of the input buffer
stages may not be precisely matched. Accordingly, the differential
amplifier stage 1102 may be provided with a tuning configuration of
three resistors R5, R6, and R120. Resistor R120 is a reference
resistor coupled between output nodes VU and VL to aid in matching
the frequency response of the output stages, and may illustratively
take a value of about 1 k.OMEGA.. Tuning resistors R5 and R6 are
select by test ("SBT") components that are tailored for each tool
to provide deep nulls in the common mode rejection response at the
frequencies of interest. In this manner, exceptional common mode
rejection can be achieved in the presence of high input source
impedances.
[0046] FIG. 12 shows a complete schematic of two input buffer
stages coupled to a differential amplifier stage, with component
values provided. The schematic also shows zero ohm resistors R123
and R 24 in the feedback paths of the input buffet stages. As
indicated by their values, these resistors are not currently part
of the preferred design. However, the selection of a low value of
resistance for R123 acts to impart a slight phase shift to the
positive feedback path, which provides an alternate means for
matching the AC responses of the input buffer stages when operating
from a very high impedance source. In some applications it may be
advantageous to utilize this method to provide deep common mode
nulls, and/or to provide good gain matching between receiver
channels in a multi-channel application.
[0047] Accordingly, there has been disclosed herein a displaced
electrode amplifier (DEA) suitable for measuring voltages in a test
object via one or more electrodes that are separated from the test
object by a layer of high impedance material. The disclosed
amplifier is also suitable for measuring voltages from other high
impedance source configurations. Single amplifier and differential
amplifier configurations are disclosed. Differential voltages can
be measured with very high common mode rejection ratios due to the
high input impedance of the disclosed amplifier and disclosed
configurations for matching frequency responses of different inputs
Positive feedback is used to compensate for parasitic shunt
components (input signal leakage paths), further increasing the
amplifier's input impedance. In some embodiments, reference
voltages are provided via a high DC impedance path, which is
further augmented by positive feedback to provide very high
impedance to AC signals, thereby minimizing loading effects for
these signals.
[0048] A variety of voltage electrode geometries are possible and
may be used. A greater number of voltage electrodes may provide
higher resolution at the expense of increased processing costs. The
operating voltages and currents may vary widely while remaining
suitable for the logging operations described herein. It has been
found that source current frequencies above about 5 kHz, and
perhaps as high as 100 kHz or more, are desirable as they reduce
the mud layer impedances and increase the voltage differences
measurable between the voltage electrodes. In some tool
embodiments, the source current frequency may be switchable between
low frequency (e.g., 10 kHz) and high frequency (e.g., 80 kHz) for
measurements in formations of differing resistivity. Higher
frequencies may be preferred for formations having a generally
lower resistivity, and vice versa.
[0049] The disclosed amplifier configuration is useful for
implementing an oil-based mud resistivity imaging tool, but its
application is not limited to this particular tool type, nor is it
limited oil-field application. It may find application in fields
where non-destructive or non-invasive testing are desired (egg,
building and highway inspections, airframe testing, medical
examinations) as well as use for measurements in hostile
environments (high-temperature, explosion hazard, or quarantine
environments) where contact measurements are undesirable or
infeasible.
[0050] While illustrative embodiments of this invention have been
shown and described, modifications thereof can be made by one
skilled in the art without departing from the spirit or teaching of
this invention. The embodiments described herein are illustrative
and are not limiting Many variations and modifications of the
system and apparatus are possible and are within the scope of the
invention. For example, though the disclosure and claims use the
term "resistivity", it is widely recognized that conductivity (the
inverse of resistivity) has a one-to-one correspondence with
resistivity and, consequently, often serves as a functional
equivalent to resistivity. Accordingly, the scope of protection is
not limited to the embodiments described herein, but is only
limited by the claims which follow, the scope of which shall
include all equivalents of the subject matter of the claims.
* * * * *